This one is a bit difficult, but it can be done. First, you'll want to get an adapter for the iMac called a Mini Displayport to DVI adapter. This plugs into the thunderbolt port and lets the iMac talk to other devices using the standard DVI cable. Then you'll need a DVI to DVI cable to plug into this adapter. Then you'll need a Mini DVI to DVI adapter to plug into the end of the DVI to DVI cord and then into the Dell monitor. So you're looking at investing in some adapters to make it work, but it can work this way.